The bomb factory in Alexandra Grove, Leeds, used by the July 7 attackers. Photograph: PA

An unidentified person in Pakistan gave bomb-making advice and encouragement to the four suicide bombers on 7 July while they were plotting the attacks that would kill 52 people, an inquest into the victims' deaths has heard.

The leader of the bombers, Mohammed Sidique Khan, received a series of calls on a mobile phone used solely to plan the attacks from a number of public call boxes in Rawalpindi, it was revealed.

The bombers are believed to have been guided by an individual in Rawalpindi because they lacked expertise in making bombs, Detective Sergeant Mark Stuart of the Metropolitan police told the inquest.

Photographs of the bomb-making factory in Leeds where the group built explosive devices using concentrated hydrogen peroxide and pepper were shown at the inquest, at the Royal Courts of Justice yesterday.

The first call from Rawalpindi was received by Khan on 23 April 2005, the inquest heard. That call was followed by a series of calls between mid-May and the start of June.

One call was made on 2 July that year, five days before the explosions, said Stuart.

Hugo Keith QC, counsel to the inquests, asked him: "Did you assess that those calls therefore were probably connected to some guidance or some means of communicating information concerned with the manufacture of the bombs and then ultimately their detonation?"

"Yes, I think they had to be," Stuart said.

Khan never made calls to Pakistan but was contacted from several phone boxes, sometimes within minutes of each other, in order that the Pakistan accomplice could conceal their identity.

"He was very careful to call only ever from public call boxes which could not be traced?" asked Keith.

"Yes, sir," replied Stuart.

The contact in Rawalpindi attempted to contact Khan for the final time at 14.04 on 7 July, five hours after he had detonated his bomb in Edgware Road, killing himself and six others.

The inquest heard that the bombers used pre-paid mobiles to plot the attacks, changing them regularly as they entered each new phase of the plan.

The bombers had used at least 19 phones during the planning and operation of the attack. After they had carried out the bombings, their families found the phones and tried calling contacts in an attempt to trace the bombers.